This invention relates generally to power-up logic reference circuits that generate one or more known logic voltage levels upon being powered up, and in particular, to a power-up logic reference circuit and related method which reliably generates one or more known voltage levels upon being powered-up, and consumes substantially no power upon being reset.
In some cases, logic integrated circuits are sensitive to their power-up logic state condition. That is, in order for this type of logic circuit to function properly, its various nodes should be at particular voltage levels upon the circuit being powered-up. However, if the logic circuit is not designed with consideration for the power-up logic levels, the various nodes may randomly or by noise acquire inappropriate voltage levels at power-up, which may cause the circuit to function improperly or not function at all. In addition, wrong voltage levels at power-up may also cause short-circuit conditions or other unwanted conditions that may cause physical damage to the circuit, such as the short-circuit condition where two push-pull transistors are both turned due to wrong voltage levels at their gates.
Often such logic circuits employ a power-up logic reference circuit that reliably generates one or more known logic voltage levels upon the circuit being powered up. The known voltage levels generated are then used by other logic circuits to set up their initial operating conditions. In this case, the power-up logic reference circuit prevents undesired voltage levels from occurring, thereby reducing the likelihood of circuit malfunction and damage due to wrong voltage levels at power-up. An example of such a power-up logic reference circuit is a resistor divider where one or more intermediate nodes generate reference voltage levels for use by other logic circuits. Typically such a circuit requires a separate reference voltage that is stable and independent of the primary circuit power supply voltage through power supply sequencing techniques.
One drawback of most prior art power-up logic reference circuits is that they typically consume power even when their use is no longer needed. That is, when the one or more logic circuits have used the reference logic levels generated by a power-up logic reference circuit in order to arrive at their initial operating condition, the power-up logic reference circuit is no longer needed. Thus, if this circuit continues to function, it unnecessarily consumes power. For example, the resistor divider previous mentioned consumes power after its reference voltage levels have been used since it continues to draw current from the power supply. Another drawback is that if an independent reference voltage is used for this purpose, the reference supply must be brought on-die and routed to the point